N-substituted maleimides are widely used as raw materials or intermediates for pharmaceuticals, agricultural chemicals, dyes, and polymer compounds. In particular, N-substituted maleimides are useful as monomers or intermediates for polymer compounds or synthetic products, and are widely used as monomers for improving heat resistance of styrene-based resins.
Various methods of preparing N-substituted maleimides are known. For example, a method of obtaining N-substituted maleimides by heating maleic acid monoamides (maleamic acids), which are easily obtainable from maleic anhydride and amines such as butylamine, octylamine, decylamine and dodecylamine, to 180° C. for effective dehydrocyclization (L. E. Coleman et al., J. Org. Chem., 24, 135(1959)) is known.
However, this method cannot be used in practice since it gives a low yield of only 15% to 50% and moreover yields a large amount of polymers of a polyamide structure as a by-product.
A known laboratory preparation method is to react maleic anhydride and aniline in the presence of a sodium acetate catalyst using a dehydrating agent such as acetic anhydride (Org. Synth. Coll., 1973, Vol. 5, page 944). Although this method gives N-substituted maleimides in a relatively high yield (75% to 80%), it has a disadvantage of high production cost since it requires the use of a stoichiometric amount of acetic anhydride, which results in additional cost of an auxiliary material.
On the other hand, dehydrocyclization of maleic acid monoamides under milder conditions using an effective dehydration catalyst without using dehydrating agents is more commercially effective. Various attempts have been made regarding such a process. There have been known, for example, a method of using a basic catalyst such as an alkali metal acetate, sodium hydroxide, or triethylamine as a catalyst (Japanese Patent Application Publication No. JP 1972-024024 B2, Canadian Patent Publication No. CA 906494 A, and German Patent Publication No. DE 02100800 A1) and a method of using an acidic catalyst such as sulfuric acid and a sulfonic acid (British Patent Publication No. GB 001041027 A). However, these methods of using such catalysts are not yet fully satisfactory in suppressing side reactions since they give polymeric products as by-products. Moreover, there is a problem that these methods require complicated steps in separation and recovery of the catalyst and removal of by-products from reaction products.
As described above, the prior art dehydrocyclization of maleic acid monoamides in the presence of a catalyst gives a relatively large amount of polymeric side-reaction products, and consequently, has drawbacks in yield, product purity, and operation procedures. Thus, suppression of the polymeric side-reaction products has become an important problem to be solved. Further, existing phosphoric acid-supported catalysts are disadvantageous in that a large amount of phosphoric acid-based components are eluted when the catalyst are reused, and as a result, their activity is drastically reduced and side reactions are greatly promoted. There is also a problem that the phosphoric acid-supported catalyst should be added periodically to maintain its reaction activity, in order to maintain the reaction for a long time.
Accordingly, it is necessary to develop a catalyst which may minimize formation of by-products during preparation of N-substituted maleimide, is reusable because its activity is not reduced even after being reused several times, and may maintain its reaction activity for a long time.